The present invention relates to an ultrasonic flaw detector for detecting flaws which may exist in the inner peripheral corner of a joint portion between the barrel of a nuclear reactor pressure vessel and a nozzle projecting from the barrel. More particularly, the invention is concerned with an ultrasonic flaw detector suitable for detecting any flaw in the inner peripheral corner of the joint portion between the cylindrical nozzle and the barrel, which has an acutely rounded outer peripheral surface of a constant radius over the entire circumference of the outer peripheral corner.
In detecting flaws which may exist in the inner peripheral corner of a joint portion between a cylindrical nozzle and the barrel portion of the pressure vessel of a nuclear reactor by means of an ultrasonic wave, according to the conventional flaw detecting method, the flaw detector is manually operated by inspectors or, alternatively, the flaw detector is mechanically guided under the supervision of at least one inspector. More specifically, according to the first-mentioned method, two inspectors are engaged in the detection under the possible influence of radioactive rays: namely one inspector who manipulates the flaw detector and one inspector who records the inspection results. In the second-mentioned method, at least one inspector has to stay in the area under the possible influence of radioactive rays for continuously supervising the operation of the flaw detector. Usually, the pressure vessel of a nuclear reactor has thirty to forty cylindrical nozzles, all of which are located in a hot area or a high activity area. It is, therefore, essential to pay specific attention to prevent excessive exposure of the inspectors to radioactive rays. In consequence, such flaw detection work in the hot area is not allowed to continue over a long time.
In addition, a visible check through the naked eyes of the inspector is essential for precisely determining amount by movement of the probe of the flaw detector. Furthermore, the direction of flaw detection by ultrasonic waves transmitted from the probe varies in complicated ways in accordance with the movement of the probe, so that the processing of the detection signal is impractically complicated.